1. Field of the Invention
The present invention relates to a constant voltage circuit for generating a constant reference voltage using a base-emitter voltage and a V.sub.T voltage.
2. Description of the Related Art
Conventionally, as a constant voltage circuit capable of setting a temperature coefficient to be 0 or an arbitrary value, a circuit disclosed in the U.S. Pat. No. 3,617,859 is well known. As shown in FIG. 1, the constant voltage circuit consists of a current source 41, three resistors 42, 43, and 44, and three npn transistors 45, 46, and 47. For example, an emitter area of the transistor 46 is N times that of the transistor 45 such that the transistor 45 and 46 have different current densities. A positive input voltage V.sub.IN and a ground voltage GND are applied to nodes 48 and 49, respectively, and an output voltage V.sub.OUT is obtained from an output node 50.
In the conventional circuit, assuming that base-emitter voltages of the transistors 45, 46, and 47 are set to be values V.sub.BE1, V.sub.BE2, and V.sub.BE3 ; resistors 42, 43, and 44 are set to have values R.sub.1, R.sub.2, and R.sub.3 ; currents flowing through the resistors 42 and 43 are set to be I.sub.1 and I.sub.2 ; and a collector current of the transistor 47 is set to be I.sub.3, the base-emitter voltage V.sub.BE1 is obtained by the following equation (1) EQU V.sub.BE1 =V.sub.BE2 +I.sub.2.R.sub.3 ( 1)
A voltage VOUT obtained from the output node 50 is given by: ##EQU1##
The base-emitter voltages V.sub.BE1, V.sub.BE2, and V.sub.BE3 of the transistors 45, 46, and 47 are expressed by the following equations: EQU V.sub.BE1 =V.sub.T ln(I.sub.1 /I.sub.s) (3) EQU V.sub.BE2 =V.sub.T ln{I.sub.2 /(N.I.sub.5)} (4) EQU V.sub.BE3 =V.sub.T ln(I.sub.3 /I.sub.s) (5)
where Is is a saturation current. Assuming that V.sub.BE1 =V.sub.BE3, the following equation can be obtained by the equation (2). EQU I.sub.1 .multidot.R.sub.1 =I.sub.2 .multidot.R.sub.2 ( 6)
The following equation can be obtained by the equations (1), (3), and (4): EQU V.sub.T ln(I.sub.1 .multidot.N/I.sub.2)=I.sub.2 .multidot.R.sub.3( 7)
When the equation (6) is substituted into the equation (7), the following equation can be obtained: EQU V.sub.T ln (R.sub.2 .multidot.N/R.sub.1)=I.sub.2 .multidot.R.sub.3( 8)
When the equation (8) is substituted into the equation (2), the following equation can be obtained: EQU V.sub.OUT =(R.sub.2 /R.sub.3).multidot.V.sub.T ln(R.sub.2 .multidot.N/R.sub.1)+V.sub.BE3 ( 9)
The equation (9) is well known as a basic equation of a bandgap voltage reference. The value V.sub.T in the equation (9) is given as a value kT/q (where: k is the Boltzman's constant; T, absolute temperature: and q, an electron charge) and has a positive temperature coefficient. Contrast to this, since the base-emitter voltage V.sub.BE3 of the transistor 47 has a negative temperature coefficient, the resistances R.sub.2 and R.sub.3 of the resistors 43 and 44 are adjusted so that the temperature coefficient of the output voltage V.sub.OUT can be set to be 0 or an arbitrary value.
Stability of the output voltage in the conventional circuit will be described below. FIG. 3 is a graph showing variation characteristics of the output voltage V.sub.OUT obtained by performing SPICE analysis when a value I of a current source is changed in a conventional circuit with the arrangement in FIG. 2. As shown in FIG. 2, the resistors 42, 43, and 44 are respectively set to be 22 K.OMEGA., 22 K.OMEGA., and 1.8 K.OMEGA., an emitter area ratio N of the transistor 45 to the transistor 46 is set to be 4. As is apparent from FIG. 3, when the current value is changed from 10 .mu.A to 30 .mu.A, an output voltage difference in the conventional circuit is 60.2 mV.
As described above, since a change in current flowing through the above conventional circuit, especially, a change in the collector current of the transistor 47 on the basis of a change in output value of the current source is not considered, the output voltage is largely changed by the change in the collector current.